Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

Now in Temporal Databases

  • Curtis E. DyresonEmail author
  • Christian S. Jensen
  • Richard T. Snodgrass
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_248


Current date; Current time; Current timestamp; Until changed


The word now is a noun in the English language that means “at the present time.” This notion appears in databases in three guises. The first use of now is as a function within queries, views, assertions, etc. For instance, in SQL, CURRENT_DATE within queries, etc., returns the current date as an SQL DATE value; CURRENT_TIME and CURRENT_TIMESTAMP are also available. These constructs are nullary functions.

In the context of a transaction that contains more than one occurrence of these functions, the issue of which time value(s) to return when these functions are invoked becomes important. When having these functions return the same (or consistent) value, it becomes a challenge to select this time and to synchronize it with the serialization time of the transaction containing the query.

The second use is as a database variable used extensively in temporal data model proposals, primarily as timestamp values...

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Recommended Reading

  1. 1.
    Ben-Zvi J. The time relational model. PhD Dissertation, University of California, Los Angeles; 1982.Google Scholar
  2. 2.
    Bliujūtė R, Jensen CS, Šaltenis S, Slivinskas G. Light-weight indexing of bitemporal data. In: Proceedings of the 12th International Conference on Scientific and Statistical Database Management; 2000. p. 125–38.Google Scholar
  3. 3.
    Chamberlin DD, Astraham MM, Eswaran KP, Griffiths PP, Lorie RA, Mehl JW, Reisner P, Wade BW. SEQUEL 2: a unified approach to data definition, manipulation, and control. IBM J Res Dev. 1976;20(6):560–75.zbMATHCrossRefGoogle Scholar
  4. 4.
    Finger M. Handling database updates in two-dimensional temporal logic. J Appl Non Classical Logics. 1992;2(2):201–24.MathSciNetzbMATHCrossRefGoogle Scholar
  5. 5.
    Clifford J, Dyreson CE, Isakowitz T, Jensen CS, Snodgrass RT. On the semantics of “now.”. ACM Trans Database Syst. 1997;22(2):171–214.CrossRefGoogle Scholar
  6. 6.
    Clifford J, Tansel AU. On an algebra for historical relational databases: two views. In: Proceedings of the ACM SIGMOD International Conference on Management of Data; 1985. p. 247–65.CrossRefGoogle Scholar
  7. 7.
    Lomet D, Snodgrass RT, Jensen CS. Exploiting the lock manager for timestamping. In: Proceedings of the International Conference on Database Engineering and Applications; 2005. p. 357–68.Google Scholar
  8. 8.
    Lorentzos NA, Johnson RG. Extending relational algebra to manipulate temporal data. Inf Syst. 1988;13(3):289–96.zbMATHCrossRefGoogle Scholar
  9. 9.
    Montague R. Formal philosophy: selected papers of Richard Montague. New Haven: Yale University Press; 1974.Google Scholar
  10. 10.
    Šaltenis S, Jensen CS. Indexing of now-relative spatio-bitemporal data. VLDB J. 2002;11(1):1–16.CrossRefGoogle Scholar
  11. 11.
    Snodgrass RT. The temporal query language TQuel. ACM Trans Database Syst. 1988;12(2):247–98.MathSciNetCrossRefGoogle Scholar
  12. 12.
    Snodgrass RT, editor. The TSQL2 Temporal Query Language. Norwell: Kluwer; 1995.zbMATHGoogle Scholar
  13. 13.
    Torp K, Jensen CS, Snodgrass RT. Modification semantics in now-relative databases. Inf Syst. 2004;29(78):653–83.CrossRefGoogle Scholar
  14. 14.
    Wiederhold G, Jajodia S, Litwin W. Integrating temporal data in a heterogeneous environment. In: Tansel A, Clifford J, Gadia SK, Jajodia S, Segev A, Snodgrass RT, editors. Temporal databases: theory, design, and implementation, Chapter 22. Redwood City: Benjamin/Cummings; 1993. p. 563–79.Google Scholar

Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  • Curtis E. Dyreson
    • 1
    Email author
  • Christian S. Jensen
    • 2
  • Richard T. Snodgrass
    • 3
    • 4
  1. 1.Utah State UniversityLoganUSA
  2. 2.Department of Computer ScienceAalborg UniversityAalborgDenmark
  3. 3.Department of Computer ScienceUniversity of ArizonaTucsonUSA
  4. 4.Dataware VenturesTucsonUSA

Section editors and affiliations

  • Richard T. Snodgrass
    • 1
  • Christian S. Jensen
    • 2
  1. 1.University of ArizonaTucsonUSA
  2. 2.Aalborg UniversityAalborg ØstDenmark